Hydrogen economy myth
The idea that hydrogen makes any sense as an energy storage system is a persistent myth. When you take into account the full life cycle only about 25% of the original energy content used to produce it will be available for use in a fuel cell. On top of that, it's bulky - even under very high pressures or even liquefaction. The very compression (or liquefaction) of hydrogen uses a very high proportion of the energy content. Because hydrogen atoms are so so small it is notoriously difficult to keep under very high pressure - it tends to percolate through liners and does bad things to the tank material over time. Diving tanks are simply unsuited to this purpose - they have to be built to a much higher level.
Only for a few uses does hydrogen make much sense - whilst it's bulky, liquid hydrogen is at least light, so it makes sense in space rockets where the energy lost in liquefying it is not the most important issue.
Pretty well nothing (at least chemically) beats the energy density of liquid hydrocarbons. It would make more sense to find an artificial way of generating those (perhaps using genetically engineered organisms and sunlight) rather than taking the high thermodynamic losses in spitting water by electrolysis and compressing or liquefying it then somehow finding a way of flying a 'plane using the stuff.
Small nuclear reactors also don't make much sense - for those to operate you need copious supplies of cooling water. That's fine in a submarine, or a ship which are, literally, surrounded by oceans of the stuff, but it's hardly going to work in a desert where even drinking water might have to be shipped in.
Here's one simple link to the basic overheads.
Yes, and for those who know about these things, there is talk of using electrolysis under high-pressure to avoid much of the compression energy costs, but it's all rather speculative.